Polymer mixtures with improved odor control

ABSTRACT

The present invention relates to polymer mixtures including hydrogel-forming polymers capable of absorbing aqueous fluids and prepared by polymerization of olefinically unsaturated carboxylic acids or derivatives thereof together with copolymers of C 2 -C 8  olefins or styrenes with anhydrides and also their preparation, use and hygiene articles which include same.

The present invention relates to polymer mixtures includinghydrogel-forming polymers capable of absorbing aqueous fluids andprepared by polymerization of olefinically unsaturated carboxylic acidsor derivatives thereof together with copolymers of C₂-C₈ olefins orstyrenes with anhydrides and also their preparation, use in hygienearticles and hygiene articles which include same. More particularly, theinvention relates to 2-component polymer mixtures of hydrogel-formingpolymers with copolymers of C₂-C₈ olefins or styrenes with anhydrides.

Swellable hydrogel-forming addition polymers, known as superabsorbentpolymers or SAPs, are known from the prior art. They are networks offlexible hydrophilic addition polymers, which can be both ionic andnonionic in nature. They are capable of absorbing and binding aqueousfluids by forming a hydrogel and therefore are preferentially used formanufacturing tampons, diapers, sanitary napkins, incontinence articles,training pants for children, insoles and other hygiene articles for theabsorption of body fluids. Superabsorbents are also used in other fieldsof technology where fluids, especially water or aqueous solutions, areabsorbed. These fields include for example storage, packaging,transportation (packaging material for water-sensitive articles, forexample flower transportation, shock protection); food sector(transportation of fish, fresh meat; absorption of water, blood in freshfish/meat packs); medicine (wound plasters, water-absorbent material forburn dressings or for other weeping wounds); cosmetics (carrier materialfor pharmaceuticals and medicaments, rheumatic plasters, ultrasound gel,cooling gel, cosmetic thickeners, sunscreen); thickeners for oil/wateror water/oil emulsions; textiles (gloves, sportswear, moistureregulation in textiles, shoe inserts); chemical process industryapplications (catalyst for organic reactions, immobilization of largefunctional molecules (enzymes), adhesive for agglomerations, heatstorage media, filtration aids, hydrophilic component in polymerlaminates, dispersants, liquefiers); building construction, installation(powder injection molding, clay-based renders, vibration-inhibitingmedium, assistants in relation to tunneling in water-rich ground, cablesheathing); water treatment, waste treatment, water removal (de-icers,reusable sandbags); cleaning; agriculture industry (irrigation,retention of meltwater and dew precipitates, composting additive,protection of forests against fungal and insect infestation, delayedrelease of active ingredients to plants); fire protection (flyingsparks)(covering houses or house walls with SAP gel, since water has avery high heat capacity, ignition can be prevented; spraying of SAP gelin the case of fires such as for example forest fires); coextrusionagent in thermoplastic polymers (hydrophilicization of multilayerfilms); production of films and thermoplastic moldings capable ofabsorbing water (for example agricultural films capable of storing rainand dew water; SAP-containing films for keeping fresh fruit andvegetables which can be packed in moist films; the SAP stores waterreleased by the fruit and vegetables without forming condensationdroplets and partly reemits the water to the fruit and vegetables, sothat neither fouling nor wilting occurs; SAP-polystyrene coextrudatesfor example for food packs such as meat, fish, poultry, fruit andvegetables); carrier substance in active-ingredient formulations (drugs,crop protection). Within hygiene articles, superabsorbents are generallypositioned in an absorbent core which, as well as SAP, comprises othermaterials, including fibers (cellulose fibers), which act as a kind ofliquid buffer to intermediately store the spontaneously applied liquidinsults and are intended to ensure efficient channelization of the bodyfluids in the absorbent core toward the superabsorbent.

The current trend in the hygiene sector, e.g. in diaper design, istoward ever thinner constructions having a reduced cellulose fibercontent and an increased hydrogel content. The trend toward ever thinnerdiaper constructions has substantially changed the performance profilerequired of the water swellable hydrophilic polymers over the years.Whereas at the start of the development of highly absorbent hydrogels itwas initially solely the very high swellability on which interestfocused, it was subsequently determined that the ability of thesuperabsorbent to transmit and distribute fluid is also of decisiveimportance. It has been determined that conventional superabsorbentsgreatly swell at the surface on wetting with liquid, so thattransportation of liquid into the particle interior is substantiallycompromised or completely prevented. This trait of superabsorbents isknown as gel blocking. The greater amount of polymer per unit area inthe hygiene article must not cause the swollen polymer to form a barrierlayer to subsequent fluid. A product having good transportationproperties will ensure optimal utilization of the entire hygienearticle. This prevents the phenomenon of gel blocking, which in theextreme case will cause the hygiene article to leak. Fluid transmissionand distribution is thus of decisive importance with regard to theinitial absorption of body fluids.

A lot of work has been done to try to generate absorbing structureswithout any additions of cellulosic fibers or other nonwoven fibrousmaterials that ideally form even continuous hydrogel zones in order toensure a higher loading of the absorbent core with highly absorptivehydrogel-forming polymer material.

The literature additionally includes accounts of the use ofwater-absorbing films which are likewise based on hydrogel-formingaddition polymers but whose monomer solution is appliedtwo-dimensionally prior to the polymerization, or else, starting fromhydrogel-forming polymer particles, an intra-molecular crosslinking iscarried out to form macrostructures. For instance, JP 04004247 describesthe preparation of a water-absorbing film from maleic anhydridecopolymer whose structural units are based on (I) alpha-olefinic orstyrene units and (II) on maleic anhydride structures.

However, the use of water-absorbing films gives rise to fluidtransportation problems, since insufficient diffusion times through thehydrogel compromise or even stop any fluid transmission into lowerlayers. The same is true of the use of hydrogel-forming polymerparticles in high concentration, where the initial swell leads to mutualcontact between the swollen hydrogel particles and hence to theformation of a gel-continuous zone.

Good transportation properties are possessed for example by hydrogelshaving high gel strength in the swollen state. Gels lacking in strengthare deformable under an applied pressure, for example pressure due tothe bodyweight of the wearer of the hygiene article, and clog the poresin the SAP/cellulosic fiber absorbent and so prevent continued uptake offluid. Enhanced gel strength is generally obtained through a higherdegree of crosslinking, although this reduces retention performance. Asmight be expected from the inherent nature of hydrogel-forming additionpolymers, it has not been possible to combine properties such as highabsorptive capacity and high gel strength in one product.

One way of enhancing gel strength while preserving high absorptivecapacities is surface postcrosslinking. In this process, driedsuperabsorbents having an average crosslink density are subjected to anadditional crosslinking step. The process is known to one skilled in theart and described in EP-A-0 349 240. Surface postcrosslinking increasesthe crosslink density in the sheath of the superabsorbent particle,whereby the absorbency under load is raised to a higher level. Whereasthe absorption capacity decreases in the superabsorbent particle sheath,the core has an improved absorption capacity (compared to the sheath)owing to the presence of mobile polymer chains, so that sheathconstruction ensures improved fluid transmission without occurrence ofthe gel blocking effect. It is perfectly desirable for the totalcapacity of the superabsorbent to be occupied not spontaneously but withtime delay. Since the hygiene article is generally repeatedly insultedwith urine, the absorption capacity of the superabsorbent shouldsensibly not be exhausted after the first disposition. However, thisleaves the problem of inadequate acquisition times, which have to beoptimized particularly in those regions of the hygiene article which areexposed to the most fluid.

When hydrogels are used in the hygiene sector, they become exposed tobody fluids such as urine or menses. Body fluids generally containmalodorous components of the amine or fatty acid type, which appearalongside the organic components anyhow present, for example, amines,acids, aldehydes, ketones, phenols, polycyclics, indoles, aromatics,polyaromatics, etc., that are responsible for unpleasant body odors.Odor development takes place in two stages, first in the course ofexudation from the body region and then when the fluid has already beenpresent in the absorption medium for a defined time. Both odor factorshave to be eliminated, since it is undesirable for cost reasons tochange the hygiene article after every absorption process.

The highly swellable hydrogels used in the hygiene sector are at presentaddition polymers having a degree of neutralization in the range from 60to 80 mol %, based on the polymerized acid-functional monomer units.However, it was found in the course of the sniff test that a higher pHwill generally encourage bacterial growth. In the process, the urea inthe urine is increasingly split by urease into carbon dioxide andammonia, which leads to a further increase in the pH. This in turnreinforces bacterial growth, and enzyme activity is further increased.One consequence of the raised pH is the occurrence of soft skin, makingthe skin more susceptible to bacterial colonization. This resultsdirectly in skin irritation which will preclude the wearing of thehygiene article for a prolonged period.

When acidic hydrogels are used in hygiene articles, odor control isgood. However, there are disadvantages with existing manufacturingprocesses, since the polymerization of the monomer solution is veryslow, so that batch operation is the only option. In addition,appreciable problems arise when it comes to dividing the fully acidicpolymer gel, and the subsequent neutralization is merelydiffusion-controlled, so that the polymer surface has an excess of base.

Hitherto the following possibilities have been available for attemptingto achieve odor control in the hygiene sector:

-   -   Odor control coupled with simultaneous absorption by addition of        inert inorganic substances having a large surface area,        generally as a solid onto the surface of powders or granules for        manufacturing absorbent polymers. Zeolites, active carbon,        bentonites, finely divided amorphous silicas such as AEROSIL® or        CAB-O-SIL® are used here.    -   Addition of substances capable of complexing with organic        molecules or with metal ions present in the body fluid to        prevent the development of unpleasant odors. This preferably        takes the form of the use of cyclodextrins (any modification of        unsubstituted cyclodextrins which contains from 6 to 12 glucose        units, for example alpha-cyclodextrin, beta-cyclodextrin,        gamma-cyclodextrin and/or derivatives and/or mixtures thereof).        Mixtures of cyclodextrins are preferred, since they provide        broader complexation of organic molecules over a wider molecular        weight range. Cyclodextrins are used in amounts from 0.1% to        about 25%, preferably from 1% to about 20%, more preferably from        2% to about 15% and especially from 3 to 10%, based on the total        weight of the composition. Cyclodextrins are added in small        particle size (usually less than 12 μm) to offer a large surface        area for odor elimination. Further complexing agents are        aminopolycarboxylic acids and their salts,        ethylenediaminetetraacetate EDTA,        ethylenediaminepentamethylenephosphonic acid,        ethylenediaminetetramethylenephosphonic acid, aminophosphates,        polyfunctional aromatics, N,N-disuccinic acid.    -   Masking of unpleasant odors by addition of perfumes or        deodorants. These are added in free form or in encapsulated form        (for example in cyclodextrins). The latter form makes it        possible to release the perfume with a time delay. Nonlimiting        examples of perfumes are allyl caproate, allylcyclohexane        acetate, allylcyclohexane propionate, allyl heptanoate, amyl        acetate, amyl propionate, anetole, anisole, benzaldehyde, benzyl        acetate, benzylacetone, benzyl alcohol, benzyl butyrate, benzyl        formate, benzyl isovalerate, benzyl propionate, butyl benzoate,        butyl caproate, camphor, cis-3-hexenyl acetate, cis-3-hexenyl        butyrate, cis-3-hexenyl caproate, cis-3-hexenyl valerate,        citronellol, citronellyl derivatives, Cyclal C, cyclohexylethyl        acetate, 2-decenal, decylaldehyde, dihydromyrcenol,        dimethylbenzylcarbinol and derivatives thereof, dimethyloctanol,        diphenyl oxide, ethyl acetate, ethyl acetoacetate, ethyl amyl        ketone, ethyl benzoate, ethyl butyrate, ethyl hexyl ketone,        ethyl phenylacetate, eucalyptol, fenchyl acetate, fenchyl        alcohol, tricyclodecenyl acetate, tricyclodecenyl propionate,        geraniol, geranyl derivatives, heptyl acetate, heptyl        isobutyrate, heptyl propionate, hexenol, hexenyl acetate,        hexenyl isobutyrate, hexyl acetate, hexyl formate, hexyl        isobutyrate, hexyl isovalerate, hexyl neopentanoate,        hydroxycitronellal, α-ionone, β-ionone, γ-ionone, isoamyl        alcohol, isobornyl acetate, isobornyl propionate, isobutyl        benzoate, isobutyl caproate, isononyl acetate, isononyl alcohol,        isomenthol, isomenthone, isononyl acetate, isopulegol,        isopulegyl acetate, isoquinoline, dodecanal, lavandulyl acetate,        ligustral, δ-limonene, linalool and derivatives, menthone,        menthyl acetate, methylacetophenone, methyl amyl ketone, methyl        anthranilate, methyl benzoate, methyl benzylacetate,        methylchavicol, methyleugenol, methylheptenone, methyl        heptynecarbonate, methyl heptyl ketone, methyl hexyl ketone,        methylnonylacetaldehyde, α-iso“γ”methylionone,        methyloctylacetaldehyde, methyl octyl ketone,        methylphenylcarbinyl acetate, methyl salicylate, myrcene,        myrcenyl acetate, neral, nerol, neryl acetate, nonalactone,        nonyl butyrate, nonyl alcohol, nonyl acetate, nonylaldehyde,        octalactone, octyl acetate, octyl alcohol, octylaldehyde,        D-limonene, p-cresol, p-cresyl methyl ether, p-cymene,        p-isopropyl-p-methylacetophenone, phenethyl anthranilate,        phenoxyethanol, phenylacetaldehyde, phenylethyl acetate,        phenylethyl alcohol, phenylethyldimethylcarbinol, α-pinene,        β-pinene, α-terpinene, γ-terpinene, terpineol, terpinyl acetate,        terpinyl propionate, tetrahydrolinalool, tetrahydromyrcenol,        thymol, prenyl acetate, propyl butyrate, pulegone, safrole,        δ-undecalactone, γ-undecalactone, undecanal, undecyl alcohol,        veratrol, verdox, vertenex, viridine.    -   Addition of urease inhibitors to control the formation or        activity of enzymes responsible for the cleavage of urea into        ammonia and hence for odor development.    -   Addition of antimicrobial substances. Enzymes control bacterial        growth and thereby minimize odor development due to bacterial        degradation processes (e.g., oxidoreductase+mediator). Examples        of antimicrobial substances include quaternary ammonium        compounds, phenols, amides, acids and nitro compounds and also        mixtures thereof.

Examples of quaternary ammonium compounds include2-(3-anilinovinyl)-3,4-dimethyloxazolinium iodide, alkylisoquinoliumbromide, benzalkonium chloride, benzethonium chloride, cetylpyridiniumchloride, chlorhexidine gluconate, chlorhexidine hydrochloride,lauryltrimethylanmonium compounds, methylbenzethonium chloride,stearyltrimethylammonium chloride, 2,4,5-trichlorophenoxide and alsomixtures thereof.

Examples of phenols include benzyl alcohol, p-chlorophenol,chlorocresol, chloroxylenol, cresol, o-cymen-5-ol (BIOSOL),hexachlorophene, chinokitiol, isopropylmethylphenol, parabens (withmethyl, ethyl, propyl, butyl, isobutyl, isopropyl, and/or sodium methylsubstituents), phenethyl alcohol, phenol, phenoxyethanol,o-phenylphenol, resorcinol, resorcinol monoacetate, sodium parabens,sodium phenolsulfonate, thioxolone, 2,4,4′-trichloro-2′-hydroxydiphenylether, zinc phenolsulfonate, di-tert-butylphenol, hydroquinone, BHT andalso mixtures thereof.

Examples of amides include diazolidinylurea, 2,4-imidazolidine-dione(HYDATOIN), 3,4,4′-trichlorocarbanilide,3-trifluoro-methyl-4,4′-dichlorocarbanilide, undecylenoic acidmonoethanol-amide and also mixtures thereof.

Examples of acids include benzoates, benzoic acid, citric acid,dehydroacetic acid, potassium sorbate, sodium citrates, sodiumdehydroacetate, sodium salicylate, sodium salicylic acid, sorbitanicacid, undecylenoic acid, zinc undecylenate, zinc oxide, zincphenolsulfonate, ascorbic acid, acetylsalicylic acid, salicylaldehyde,salicylic acid derivatives, adipic acid, adipic acid derivatives andalso mixtures thereof.

Examples of nitro compounds include 2-bromo-2-nitro-2,3-propanediol(BRONOPOL), methyldibromoglutaronitrile and propylene glycol (MERGUARD)and also mixtures thereof.

In addition the following compounds are useful as biocides:2,5-dimethoxytetrahydrofuran, 2,5-diethoxytetrahydrofuran,2,5-dimethoxy-2,5-dihydrofuran, 2,5-diethoxy-2,5-dihydrofuran,succinaldehyde, glutaraldehyde, glyoxal, glyoxylic acid,hexahydrotriazine, tetrahydro-3,5-dimethyl-2H-1,3,5-thiadiazine-2-thione(Dazomet), 2,4-dichlorobenzyl alcohol, benzalkonium chloride,chlorhexidine gluconate, triclosan.

-   -   Use of transition metal compounds (Cu, Ag, Zn). Use of        microcapsules which release the active substance on contact with        moisture.

As well as the classes of compounds mentioned, useful odor controlcompounds further include the following: peroxides, bicarbonate,triclosan, plant extracts, ethereal oils, boron compounds,poly-alpha-amino acids (polylysine), imides, polyimides, PVP-iodine, useof certain polymeric substances such as chitosan, polyglycosides,oxidizing agents, cyclophanes.

In general, however, the addition of odor inhibitors will have anadverse effect on the absorption profile of superabsorbent hydrogels.Therefore, the polymer mixtures of the present invention are preferablyused without these odor-inhibiting materials.

It is an object of the present invention to develop a product combininghigh absorptive performance and/or swell rate with odor-bindingproperties.

We have found that this object is achieved, surprisingly, on admixingcustomary hydrogel-forming addition polymers with copolymers of C₂-C₈olefins or styrenes with anhydrides.

The present invention accordingly provides polymer mixtures includinghydrogel-forming polymers capable of absorbing aqueous fluids andprepared by polymerization of olefinically unsaturated carboxylic acidsor derivatives thereof (component (i)), with copolymers of C₂-C₈ olefinsor styrenes with anhydrides (components(ii)). The polymer mixtures ofthe present invention preferably comprise granular or fibroushydrogel-forming polymers capable of absorbing aqueous fluids andprepared by polymerization of olefinically unsaturated carboxylic acidsor derivatives thereof in combination with granular or fibrouscopolymers of C₂-C₈ olefins or styrenes with anhydrides. Alternatively,the copolymers of C₂-C₈ olefins or styrenes with anhydrides can also besprayed onto the granular or fibrous hydrogel-forming polymers capableof absorbing aqueous fluids and prepared by polymerization ofolefinically unsaturated carboxylic acids or derivatives thereof. Theexpression “granular or fibrous hydrogel-forming polymers capable ofabsorbing aqueous fluids and prepared by polymerization of olefinicallyunsaturated carboxylic acids or derivatives thereof” as used in thepresent invention does not comprehend copolymers of C₂-C₈ olefins orstyrenes with anhydrides. The hydrogel-forming polymers capable ofabsorbing aqueous fluids are preferably prepared by polymerization ofacrylic acid or salts thereof. These hydrogel-forming polymers capableof absorbing aqueous fluids are therefore preferably based onpolyacrylate. The copolymers of C₂-C₈ olefins or styrenes withanhydrides can themselves be hydrogel-forming, for example by beingpartially hydrolyzed. In that case, they have preferably been hydrolyzedto 15 mol % or less. Particular preference is given to unhydrolyzedcopolymers. Preference is given to polymer mixtures prepared by aprocess in which the hydrogel-forming polymers and the copolymer ofC₂-C₈ olefins or styrenes with anhydrides are prepared in two steps andsubsequently mixed in a defined ratio.

The copolymers of C₂-C₈ olefins or styrenes with anhydrides arepreferably used unhydrolyzed in pulverulent (granular) form. When thiscopolymer is used in the hygiene sector, the anhydride componentring-opens to take up the basic components (ammonia for example) whichare substantially responsible for odor development and which are formedby enzymatic processes or bacterial degradation reactions. For thepurposes of the present invention, the term “anhydride component”preferably comprehends anhydrides of olefinically unsaturated di- orpolycarboxylic acids, for example maleic acid substituted by one or twoC₁-C₆-alkyl groups. Dicarboxylic acids are preferred. Maleic anhydrideis particularly preferred. By C₂-C₈ olefins are meant unsaturatedcompounds containing from 2 to 8 carbon atoms. Optionally, they may alsocontain one or more heteroatoms such as O, N, S. The following monomersare contemplated by way of example: ethylene, propylene, isobutylene,1-butylene, C₁-C₄-methacrylates, vinyl acetate, methyl vinyl ether,isobutyl vinyl ether, 1-hexene. The monomers can be pure or mixed.Preference is given to isobutylene and vinyl acetate. By styrenes aremeant styrene and substituted styrenes. There can be for example aC₁-C₆-alkyl group on the alpha carbon and/or the benzene ring can besubstituted by one or more C₁-C₆-alkyl groups and/or one or morehydroxyl groups. Styrene is preferred. The molar ratio between anhydrideand olefin or styrene is generally in the range from 3:1 to 1:3,preferably in the range from 2.5:1 to 1:2.5, more preferably in therange from 2:1 to 1:2 and especially 1:1. If there is an excess of onecomponent, an excess of anhydride is preferred. The copolymers withmaleic anhydride and isobutylene, diisobutylene, ethylene or styrene,all optionally with addition of vinyl acetate, are particularlypreferred.

It has also been determined that, surprisingly, the addition of granularor fibrous copolymers of C₂-C₈ olefins or styrenes with anhydrides tothe hydrogel-forming polymers of the prior art substantially increasesthe permeability.

The hydrogel-forming polymers capable of absorbing aqueous fluids canalso be admixed with monomer solution or polymer solution of C₂-C₈olefins or styrenes with anhydrides. Useful solvents include inertsolvents such as acetone, DMSO, dioxane, ethyl acetate, chloroform andtoluene. The monomer or polymer solutions can be sprayed onto thehydrogels and, if appropriate, subjected to a precipitationpolymerization. The solvent is advantageously removed by drying at from20 to 120° C.

The molar masses of the copolymers are generally in the range from 500to 1 million and preferably in the range from 1 000 to 250 000.

The fraction of hydrogel-forming polymer particles exhibits a highabsorptive capacity coupled with good swell rate, while the obtention ofthe anhydride units ensures the buffering of the fraction of basiccomponents, for example of the ammonia fraction. The addition ofcopolymers of C₂-C₈ olefins or styrenes with anhydrides, moreover,provides improved permeability.

In a preferred embodiment of the present invention, the hydrogel-formingpolymers are admixed with fibrous copolymers of C₂-C₈ olefins orstyrenes with anhydrides. To permit spinning, the copolymer of C₂-C₈olefins or styrenes with anhydrides is partially hydrolyzed. Suchpartially neutralized copolymers of C₂-C₈ olefins or styrenes withanhydrides are described for example in U.S. Pat. No. 5,026,784.According to this reference, an aqueous, fiberizable copolymer solutionis obtained from (column 3, line 6) a) partially neutralized C₂-C₈olefin-anhydride, especially maleic anhydride, copolymer having a degreeof neutralization in the range from 0.2 to about 0.8 equivalent ofcarboxyl group units with b) from 0.1 to 10 parts by weight of at leastone reactive component per 100 parts by weight of partially neutralizedpolymer dissolved in aqueous fluid. The reactive component is awater-soluble component bearing one amino group and at least onehydroxyl group. The reaction product has an ionic ammonium carboxylatebond formed by unneutralized carboxyl groups on the polymer and theamino group on the reactive substance.

After spinning, the fibers are heated to 140-210° C. to cure them byremoving water and crosslinking through ester and amide linkages. Thefibers thus crosslinked are water swellable and hence absorbent.

The addition of partially neutralized copolymers of C₂-C₈ olefins orstyrenes with anhydrides in fiber form can provide higher acquisitionrates and higher retention values than is the case with the granularpolymer mixture of the present invention. A partial hydrolysis consumesanhydride groups and therefore generally either more copolymer is usedand/or known odor inhibitors are added. The degree of partial hydrolysisis preferably not more than 15 mol %.

A particularly preferred embodiment of the present invention accordinglyconcerns polymer mixtures including hydrogel-forming polymers capable ofabsorbing aqueous fluids and prepared by polymerization of olefinicallyunsaturated carboxylic acids or their derivatives with copolymers ofC₂-C₈ olefins or styrenes with anhydrides (granular or by spraying) andalso partially hydrolyzed C₂-C₈ olefin-anhydride, especially maleicanhydride, copolymer fibers.

The polymer mixtures can be mixtures of dry hydrogel-forming polymerscapable of absorbing aqueous fluids and preparable by polymerization ofolefinically unsaturated carboxylic acids or derivatives thereof withdry granular copolymers of C₂-C₈ olefins or styrenes with anhydrides.The former can also have a residual water content which is lower thantheir respective CRC. The residual moisture content is preferably lowerthan the intrinsic weight of the superabsorbent, more preferably lowerthan 30% by weight of residual moisture, especially less than 10% byweight of residual moisture. Olefinically unsaturated carboxylic acidsare preferably monoethylenically unsaturated monomers. The term“derivatives thereof” comprehends salts, esters, for example C₁-C₆-alkylesters, anhydrides, etc, which are hydrolyzable to the free acids.

Preferred polymer mixtures are characterized in that they arepulverulent mixtures of hydrogel-forming polymers capable of absorbingaqueous fluids (component (i)) with copolymers of C₂-C₈ olefins orstyrenes with anhydrides (component (ii)).

Preference is likewise given to polymer mixtures wherein the component(i) is present in a fraction in the range from 99.7% by weight to 85% byweight and the component (ii) is present in a fraction in the range from0.3% by weight to 15% by weight, especially those wherein component (ii)is present in a fraction in the range from 0.5% by weight to 10% byweight, ie for example in 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1% by weightor in between weight percentages. The remaining component or componentsthen add to make 100% by weight in each case.

In a particularly preferred embodiment of the present invention,component (ii) comprises mixtures of granular C₂-C₈ olefin-anhydride,especially maleic anhydride, copolymer and C₂-C₈ olefin-anhydride,especially maleic anhydride, copolymer fibers capable of absorbingaqueous fluids.

Preference is likewise given to polymer mixtures wherein granular C₂-C₈olefin-anhydride, especially maleic anhydride, copolymer (A) and C₂-C₈olefin-anhydride, especially maleic anhydride, copolymer fibers (B)capable of absorbing aqueous fluids are present as the two constituentsof component (ii) in a constituent (A) fraction of from 50% by weight to90% by weight and a constituent (B) fraction of from 10% by weight to50% by weight or a weight percentage in between. The remaining componentor components then combine in each case with the two main constituentsto add up to 100% by weight.

The polymer mixtures mentioned can be characterized in that, accordingto application, the components of the mixture are prepared fromparticles of the same or different particle size fraction. They arepreferably of the same particle size fraction.

The individual components are mixed after the optional surfacepostcrosslinking of component (i).

The present invention also discloses various applications for thepolymer mixtures as an absorbent for aqueous fluids, dispersions andemulsions, especially various hygiene article constructions containingthe above polymer mixtures. Particular preference is given to the use ofthe polymer mixtures of the present invention as an absorbent foraqueous fluids that provides reduced odor formation. Reduced odorformation means that the addition of the copolymers at 10% by weightimproves the buffering capacity by at least 0.2 pH unit, preferably atleast 0.5 pH unit, more preferably at least 0.7 pH unit and especiallyat least 1.0 pH unit.

Methods of Making Hydrogel-Forming Polymers

The water-swellable hydrophilic hydrogel-forming polymers are generallyprepared by free-radical polymerization in an aqueous solution whichincludes the monomers and also, if appropriate, grafting base andcrosslinkers.

Monomers Used

Hydrogel-forming polymers are in particular polymers of (co)polymerizedhydrophilic monomers, graft (co)polymers of one or more hydrophilicmonomers on a suitable grafting base, crosslinked cellulose or starchethers, crosslinked carboxymethylcellulose, partially crosslinkedpolyalkylene oxide or natural products that swell in aqueous fluids, forexample guar derivatives, alginates and carrageenans.

Suitable grafting bases can be of natural or synthetic origin. Examplesare starch, cellulose or cellulose derivatives, such ascarboxymethylcellulose, and also other polysaccharides andoligosaccharides, polyvinyl alcohol, polyalkylene oxides, especiallypolyethylene oxides and polypropylene oxides, polyamines, polyamides andalso hydrophilic polyesters. Suitable polyalkylene oxides have forexample the formula R¹—O—(CH₂—CHX—O)_(n)—R² where

-   -   R¹ and R² are independently hydrogen, alkyl, alkenyl or aryl,    -   x is hydrogen or methyl and    -   n is an integer from 1 to 10 000.    -   R¹ and R² are each preferably hydrogen, (C₁-C₄)-alkyl,        (C₂-C₆)-alkenyl or phenyl.

Preferred hydrogel-forming polymers are crosslinked polymers having acidgroups which are predominantly in the form of their salts, generallyalkali metal or ammonium salts. Such polymers swell particularlystrongly on contact with aqueous fluids to form gels.

Preference is given to polymers which are obtained by crosslinkingpolymerization or copolymerization of acid-functional monoethylenicallyunsaturated monomers or derivatives thereof, eg salts, esters,anhydrides. It is further possible to (co)polymerize these monomerswithout crosslinker and to crosslink them subsequently.

Examples of such monomers bearing acid groups are monoethylenicallyunsaturated C₃- to C₂₅-carboxylic acids or anhydrides such as acrylicacid, methacrylic acid, ethacrylic acid, α-chloroacrylic acid, crotonicacid, maleic acid, maleic anhydride, itaconic acid, citraconic acid,mesaconic acid, glutaconic acid, aconitic acid and fumaric acid. It isalso possible to use monoethylenically unsaturated sulfonic orphosphonic acids, for example vinylsulfonic acid, allylsulfonic acid,sulfoethyl acrylate, sulfo methacrylate, sulfopropyl acrylate,sulfopropyl methacrylate, 2-hydroxy-3-acryloyloxypropylsulfonic acid,2-hydroxy-3-methacryloyloxypropylsulfonic acid, vinylphosphonic acid,allylphosphonic acid, styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid. The monomers may be usedalone or mixed.

Preferred monomers used are acrylic acid, methacrylic acid,vinylsulfonic acid, acrylamidopropanesulfonic acid or mixtures thereof,for example mixtures of acrylic acid and methacrylic acid, mixtures ofacrylic acid and acrylamidopropanesulfonic acid or mixtures of acrylicacid and vinylsulfonic acid.

To optimize properties, it can be sensible to use additionalmonoethylenically unsaturated compounds which do not bear an acid groupbut are copolymerizable with the monomers bearing acid groups. Suchcompounds include for example the amides and nitriles ofmonoethylenically unsaturated carboxylic acids, for example acrylamide,methacrylamide and N-vinylformamide, N-vinylacetamide,N-methyl-N-vinylacetamide, acrylonitrile and methacrylonitrile. Examplesof further suitable compounds are vinyl esters of saturated C₁- toC₄-carboxylic acids such as vinyl formate, vinyl acetate or vinylpropionate, alkyl vinyl ethers having at least 2 carbon atoms in thealkyl group, for example ethyl vinyl ether or butyl vinyl ether, estersof monoethylenically unsaturated C₃- to C₆-carboxylic acids, for exampleesters of monohydric C₁- to C₁₈-alcohols and acrylic acid, methacrylicacid or maleic acid, monoesters of maleic acid, for example methylhydrogen maleate, N-vinyllactams such as N-vinylpyrrolidone orN-vinylcaprolactam, acrylic and methacrylic esters of alkoxylatedmonohydric saturated alcohols, for example of alcohols having from 10 to25 carbon atoms which have been reacted with from 2 to 200 mol ofethylene oxide and/or propylene oxide per mole of alcohol, and alsomonoacrylic esters and monomethacrylic esters of polyethylene glycol orpolypropylene glycol, the molar masses (M_(n)) of the polyalkyleneglycols being up to 2 000, for example. Further suitable monomers arestyrene and alkyl-substituted styrenes such as ethylstyrene ortert-butylstyrene.

These monomers without acid groups may also be used in mixture withother monomers, for example mixtures of vinyl acetate and 2-hydroxyethylacrylate in any proportion. These monomers without acid groups are addedto the reaction mixture in amounts within the range from 0 to 50% byweight, preferably less than 20% by weight.

Preference is given to crosslinked polymers of monoethylenicallyunsaturated monomers which bear acid groups and which are optionallyconverted into their alkali metal or ammonium salts before or afterpolymerization and of 0-40% by weight, based on their total weight, ofmonoethylenically unsaturated monomers which do not bear acid groups.

Preference is given to crosslinked polymers of monoethylenicallyunsaturated C₃- to C₁₂-carboxylic acids and/or their alkali metal orammonium salts. Preference is given in particular to crosslinkedpolyacrylic acids where 5-30 mol %, preferably 5-20 mol % andparticularly preferably 5-10 mol % of their acid groups, based on themonomers containing acid groups, are present as alkali metal or ammoniumsalts.

Possible crosslinkers include compounds containing at least twoethylenically unsaturated double bonds. Examples of compounds of thistype are N,N′-methylenebisacrylamide, polyethylene glycol diacrylatesand polyethylene glycol dimethacrylates each derived from polyethyleneglycols having a molecular weight of from 106 to 8 500, preferably from400 to 2 000, trimethylolpropane triacrylate, ethoxylatedtrimethylolpropane triacrylate (ETMPTA) especially ETMPTA ethoxylatedwith 15 EO on average, trimethylolpropane trimethacrylate, ethyleneglycol diacrylate, ethylene glycol dimethacrylate, propylene glycoldiacrylate, propylene glycol dimethacrylate, butanediol diacrylate,butanediol dimethacrylate, hexanediol diacrylate, hexanedioldimethacrylate, allyl methacrylate, diacrylates and dimethacrylates ofblock copolymers of ethylene oxide and propylene oxide, polyhydricalcohols, such as glycerol or pentaerythritol, doubly or more highlyesterified with acrylic acid or methacrylic acid, triallylamine,dialkyldiallylammonium halides such as dimethyldiallylammonium chlorideand diethyldiallylammonium chloride, tetraallylethylenediamine,divinylbenzene, diallyl phthalate, polyethylene glycol divinyl ethers ofpolyethylene glycols having a molecular weight of from 106 to 4 000,trimethylolpropane diallyl ether, butanediol divinyl ether,pentaerythritol triallyl ether, reaction products of 1 mol of ethyleneglycol diglycidyl ether or polyethylene glycol diglycidyl ether with 2mol of pentaerythritol triallyl ether or allyl alcohol, and/ordivinylethyleneurea. Preference is given to using water-solublecrosslinkers, for example N,N′-methylenebisacrylamide, polyethyleneglycol diacrylates and polyethylene glycol dimethacrylates derived fromaddition products of from 2 to 400 mol of ethylene oxide with 1 mol of adiol or polyol, vinyl ethers of addition products of from 2 to 400 molof ethylene oxide with 1 mol of a diol or polyol, ethylene glycoldiacrylate, ethylene glycol dimethacrylate or triacrylates andtrimethacrylates of addition products of from 6 to 20 mol of ethyleneoxide with 1 mol of glycerol, pentaerythritol triallyl ether and/ordivinylurea.

Possible crosslinkers also include compounds containing at least onepolymerizable ethylenically unsaturated group and at least one furtherfunctional group. The functional group of these crosslinkers has to becapable of reacting with the functional groups, essentially the acidgroups, of the monomers. Suitable functional groups include for examplehydroxyl, amino, epoxy and aziridino groups. Useful are for examplehydroxyalkyl esters of the abovementioned monoethylenically unsaturatedcarboxylic acids, e.g., 2-hydroxyethyl acrylate, hydroxypropyl acrylate,hydroxybutyl acrylate, hydroxyethyl methacrylate, hydroxypropylmethacrylate and hydroxybutyl methacrylate, allylpiperidinium bromide,N-vinylimidazoles, for example N-vinylimidazole,1-vinyl-2-methylimidazole and N-vinylimidazolines such asN-vinylimidazoline, 1-vinyl-2-methylimidazoline,1-vinyl-2-ethylimidazoline or 1-vinyl-2-propylimidazoline, which can beused in the form of the free bases, in quaternized form or as salt inthe polymerization. It is also possible to use dialkylaminoethylacrylate and dimethylaminoethyl methacrylate, diethylaminoethyl acrylateand diethylaminoethyl methacrylate. The basic esters are preferably usedin quaternized form or as salt. It is also possible to useglycidyl(meth)acrylate, for example.

Useful crosslinkers further include compounds containing at least twofunctional groups capable of reacting with the functional groups,essentially the acid groups, of the monomers. Suitable functional groupswere already mentioned above, ie, hydroxyl, amino, epoxy, isocyanato,ester, amido and aziridino groups. Examples of such crosslinkers areethylene glycol, diethylene glycol, triethylene glycol, tetraethyleneglycol, polyethylene glycol, glycerol, polyglycerol, triethanolamine,propylene glycol, polypropylene glycol, block copolymers of ethyleneoxide and propylene oxide, ethanolamine, sorbitan fatty acid esters,ethoxylated sorbitan fatty acid esters, trimethylolpropane,pentaerythritol, 1,3-butanediol, 1,4-butanediol, polyvinyl alcohol,sorbitol, starch, polyglycidyl ethers such as ethylene glycol diglycidylether, polyethylene glycol diglycidyl ether, glycerol diglycidyl ether,glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerolpolyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritolpolyglycidyl ether, propylene glycol diglycidyl ether and polypropyleneglycol diglycidyl ether, polyaziridine compounds such as2,2-bishydroxymethylbutanol tris[3-(1-aziridinyl)propionate],1,6-hexamethylenediethyleneurea,diphenylmethanebis-4,4′-N,N′-diethyleneurea, haloepoxy compounds such asepichlorohydrin and α-methylepifluorohydrin, polyisocyanates such as2,4-toluylene diisocyanate and hexamethylene diisocyanate, alkylenecarbonates such as 1,3-dioxolan-2-one and 4-methyl-1,3-dioxolan-2-one,also bisoxazolines and oxazolidones, polyamidoamines and also theirreaction products with epichlorohydrin, also polyquaternary amines suchas condensation products of dimethylamine with epichlorohydrin, homo-and copolymers of diallyldimethylammonium chloride and also homo- andcopolymers of dimethylaminoethyl(meth)acrylate which are optionallyquaternized with, for example, methyl chloride.

Useful crosslinkers further include multivalent metal ions capable offorming ionic crosslinks. Examples of such crosslinkers are magnesium,calcium, barium and aluminum ions. These crosslinkers are used forexample as hydroxides, carbonates or bicarbonates. Useful crosslinkersfurther include multifunctional bases likewise capable of forming ioniccrosslinks, for example polyamines or their quaternized salts. Examplesof polyamines are ethylenediamine, diethylenetriamine,triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine andpolyethyleneimines and also polyamines having molar masses in each caseof up to 4 000 000.

The crosslinkers are present in the reaction mixture for example from0.001 to 20% and preferably from 0.01 to 14% by weight, based onmonomer.

Free Radical Polymerization

The polymerization is initiated in the generally customary manner, bymeans of an initiator. But the polymerization may also be initiated byelectron beams acting on the polymerizable aqueous mixture. However, thepolymerization may also be initiated in the absence of initiators of theabovementioned kind, by the action of high energy radiation in thepresence of photoinitiators. Useful polymerization initiators includeall compounds which decompose into free radicals under thepolymerization conditions, for example peroxides, hydroperoxides,hydrogen peroxides, persulfates, azo compounds and redox catalysts. Theuse of water-soluble initiators is preferred. In some cases it isadvantageous to use mixtures of different polymerization initiators, forexample mixtures of hydrogen peroxide and sodium peroxodisulfate orpotassium peroxodisulfate. Mixtures of hydrogen peroxide and sodiumperoxodisulfate may be used in any proportion. Examples of suitableorganic peroxides are acetylacetone peroxide, methyl ethyl ketoneperoxide, tert-butyl hydroperoxide, cumene hydroperoxide, tert-amylperpivalate, tert-butyl perpivalate, tert-butyl perneohexanoate,tert-butyl perisobutyrate, tert-butyl per-2-ethylhexanoate, tert-butylperisononanoate, tert-butyl permaleate, tert-butyl perbenzoate,di(2-ethylhexyl)peroxydicarbonate, dicyclohexyl peroxydicarbonate,di(4-tert-butylcyclohexyl)peroxydicarbonate, dimyristylperoxydicarbonate, diacetyl peroxydicarbonate, allyl peresters, cumylperoxyneodecanoate, tert-butyl per-3,5,5-trimethylhexanoate,acetylcyclohexylsulfonyl peroxide, dilauryl peroxide, dibenzoyl peroxideand tert-amyl perneodecanoate. Particularly suitable polymerizationinitiators are water-soluble azo initiators, e.g.,2,2′-azobis(2-amidino-propane)dihydrochloride,2,2′-azobis(N,N′-dimethylene)-isobutyramidine dihydrochloride,2-(carbamoylazo)isobutyro-nitrile,2,2′-azobis[2-(2′-imidazolin-2-yl)propane]dihydrochloride and4,4′-azobis(4-cyanovaleric acid). The polymerization initiatorsmentioned are used in customary amounts, for example in amounts of from0.01 to 5%, preferably from 0.05 to 2.0%, by weight, based on themonomers to be polymerized.

Useful initiators also include redox catalysts. In redox catalysts, theoxidizing component is at least one of the above-specified per compoundsand the reducing component is for example ascorbic acid, glucose,sorbose, ammonium or alkali metal bisulfite, sulfite, thiosulfate,hyposulfite, pyrosulfite or sulfide, or a metal salt, such as iron(II)ions or sodium hydroxymethylsulfoxylate. The reducing component in theredox catalyst is preferably ascorbic acid or sodium sulfite. Based onthe amount of monomers used in the polymerization, from 3×10⁻⁶ to 1 mol% may be used for the reducing component of the redox catalyst systemand from 0.001 to 5.0 mol % for the oxidizing component of the redoxcatalyst, for example.

When the polymerization is initiated using high energy radiation, theinitiator used is customarily a photoinitiator. Photoinitiators includefor example α-splitters, H-abstracting systems or else azides. Examplesof such initiators are benzophenone derivatives such as Michler'sketone, phenanthrene derivatives, fluorene derivatives, anthraquinonederivatives, thioxanthone derivatives, coumarin derivatives, benzoinethers and derivatives thereof, azo compounds such as the abovementionedfree-radical formers, substituted hexaarylbisimidazoles or acylphosphineoxides. Examples of azides are: 2-(N,N-dimethylamino)ethyl4-azidocinnamate, 2-(N,N-dimethylamino)ethyl 4-azidonaphthyl ketone,2-(N,N-dimethylamino)ethyl 4-azidobenzoate, 5-azido-1-naphthyl2′-(N,N-dimethylamino)ethyl sulfone,N-(4-sulfonylazidophenyl)-maleimide, N-acetyl-4-sulfonylazidoaniline,4-sulfonyl-azidoaniline, 4-azidoaniline, 4-azidophenacyl bromide,p-azidobenzoic acid, 2,6-bis(p-azidobenzylidene)cyclohexanone and2,6-bis(p-azidobenzylidene)-4-methylcyclohexanone. Photoinitiators, ifused, are customarily used in amounts of from 0.01 to 5% of the weightof the monomers to be polymerized.

The crosslinked polymers are preferably used in partially neutralizedform. The degree of neutralization is preferably in the range from 5 to60 mol %, more preferably in the range from 10 to 40 mol %, particularlypreferably in the range from 20 to 30 mol %, based on the monomerscontaining acid groups. Useful neutralizing agents include alkali metalbases or ammonia/amines. Preference is given to the use of aqueoussodium hydroxide solution, aqueous potassium hydroxide solution orlithium hydroxide. However, neutralization may also be effected usingsodium carbonate, sodium bicarbonate, potassium carbonate or potassiumbicarbonate or other carbonates or bicarbonates or ammonia. Moreoverprimary, secondary and tertiary amines may be used.

Alternatively, the degree of neutralization can be set before, during orafter the polymerization in all apparatuses suitable for this purpose.The neutralization can be effected for example directly in a kneaderused for the polymerization. The varying degree of neutralizationentails different pH values on the part of the polymers.

Industrial processes useful for making these products include allprocesses which are customarily used to make superabsorbents, asdescribed for example in Chapter 3 of “Modern Superabsorbent PolymerTechnology”, F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998.

Polymerization in aqueous solution is preferably conducted as a gelpolymerization. It involves 10-70% strength by weight aqueous solutionsof the monomers and optionally of a suitable grafting base beingpolymerized in the presence of a free-radical initiator by utilizing theTrommsdorff-Norrish effect.

The polymerization reaction may be carried out at from 0 to 150° C.,preferably at from 10 to 100° C., not only at atmospheric pressure butalso at superatmospheric or reduced pressure. As is customary, thepolymerization may also be conducted in a protective gas atmosphere,preferably under nitrogen.

By subsequently heating the polymer gels at from 50 to 130° C.,preferably at from 70 to 100° C., for several hours, the performancecharacteristics of the polymers can be further improved.

Surface Postcrosslinking

Preference is given to hydrogel-forming polymers which have beensurface-postcrosslinked. Surface postcrosslinking may be carried out ina conventional manner using dried, ground and classified polymerparticles.

To effect surface postcrosslinking, compounds capable of reacting withthe functional groups of the polymers by crosslinking are applied to thesurface of the hydrogel particles, preferably in the form of an aqueoussolution. The aqueous solution may contain water-miscible organicsolvents. Suitable solvents are alcohols such as methanol, ethanol,i-propanol ethylene glycol, propylene glycol or acetone.

The subsequent crosslinking reacts polymers which have been prepared bythe polymerization of the abovementioned monoethylenically unsaturatedacids and optionally monoethylenically unsaturated comonomers and whichhave a molecular weight of greater than 5 000, preferably greater than50 000, with compounds which have at least two groups reactive towardacid groups. This reaction can take place at room temperature or else atelevated temperatures up to 220° C.

Suitable postcrosslinkers include for example:

-   -   di- or polyglycidyl compounds such as diglycidyl phosphonates or        ethylene glycol diglycidyl ether, bischlorohydrin ethers of        polyalkylene glycols,    -   alkoxysilyl compounds,    -   polyaziridines, aziridine compounds based on polyethers or        substituted hydrocarbons, for example bis-N-aziridinomethane,    -   polyamines or polyamidoamines and their reaction products with        epichlorohydrin,    -   polyols such as ethylene glycol, 1,2-propanediol,        1,4-butanediol, glycerol, methyltriglycol, polyethylene glycols        having an average molecular weight M_(w) of 200-10 000, di- and        polyglycerol, pentaerythritol, sorbitol, the ethoxylates of        these polyols and their esters with carboxylic acids or carbonic        acid such as ethylene carbonate or propylene carbonate,    -   carbonic acid derivatives such as urea, thiourea, guanidine,        dicyandiamide, 2-oxazolidinone and its derivatives,        bisoxazoline, polyoxazolines, di- and polyisocyanates,    -   di- and poly-N-methylol compounds such as, for example,        methylenebis(N-methylolmethacrylamide) or melamine-formaldehyde        resins,    -   compounds having two or more blocked isocyanate groups such as,        for example, trimethylhexamethylene diisocyanate blocked with        2,2,3,6-tetramethylpiperidin-4-one.

If necessary, acidic catalysts may be added, for examplep-toluenesulfonic acid, phosphoric acid, boric acid or ammoniumdihydrogenphosphate.

Particularly suitable postcrosslinkers are di- or polyglycidyl compoundssuch as ethylene glycol diglycidyl ether, the reaction products ofpolyamidoamines with epichlorohydrin and 2-oxazolidinone.

The crosslinker solution is preferably applied to the particles byspraying with a solution of the crosslinker in conventional reactionmixers or mixing and drying equipment such as Patterson-Kelly mixers,DRAIS turbulence mixers, Lödige mixers, screw mixers, plate mixers,fluidized bed mixers and Schugi Mix. The spraying of the crosslinkersolution may be followed by a heat treatment step, preferably in adownstream dryer, at from 80 to 230° C., preferably 80-190° C.,particularly preferably at from 100 to 160° C., for from 5 minutes to 6hours, preferably from 10 minutes to 2 hours, particularly preferablyfrom 10 minutes to 1 hour, during which not only cracking products butalso solvent fractions can be removed. But the drying may also takeplace in the mixer itself, by heating the jacket or by blowing in apreheated carrier gas.

In a particularly preferred embodiment of the invention, thehydrophilicity of the particle surface of the hydrogel-forming polymeris additionally modified by formation of complexes. The formation ofcomplexes on the outer shell of the hydrogel particles is effected byspraying with solutions of divalent or more highly valent metal saltsolutions, and the metal cations can react with the acid groups of thepolymer to form complexes. Examples of divalent or more highly valentmetal cations are Mg²⁺, Ca²⁺, Al³⁺, Sc³⁺, Ti⁴⁺, Mn²⁺, Fe^(2+/3+), Co²⁺,Ni²⁺, Cu^(+/2+), Zn²⁺, Y³⁺, Zr⁴⁺, Ag⁺, La³⁺, Ce⁴⁺, Hf⁴⁺, and Au^(+/3+),preferred metal cations are Mg²⁺, Ca²⁺, Al³⁺, Ti⁴⁺, Zr⁴⁺and La³⁺, andparticularly preferred metal cations are Al³⁺, Ti⁴⁺and Zr⁴⁺. The metalcations may be used not only alone but also mixed with each other. Ofthe metal cations mentioned, all metal salts are suitable that possessadequate solubility in the solvent to be used. Of particular suitabilityare metal salts with weakly complexing anions such as for examplechloride, nitrate and sulfate. Useful solvents for the metal saltsinclude water, alcohols, DMF, DMSO and also mixtures thereof. Particularpreference is given to water and water-alcohol mixtures such as forexample water-methanol or water-1,2-propanediol.

The spraying of the metal salt solution onto the particles of thehydrogel-forming polymer may be effected not only before but also afterthe surface postcrosslinking of the particles. In a particularlypreferred process, the spraying of the metal salt solution takes placein the same step as the spraying of the crosslinker solution, the twosolutions being sprayed separately in succession or simultaneously viatwo nozzles or the crosslinker and metal salt solutions may be sprayedconjointly through a single nozzle.

Optionally, the hydrogel-forming polymers may be further modified byadmixture of finely divided inorganic solids, for example silica,alumina, titanium dioxide and iron(II) oxide, to further augment theeffects of the surface aftertreatment. Particular preference is given tothe admixture of hydrophilic silica or of alumina having an averageprimary particle size of from 4 to 50 nm and a specific surface area of50-450 m²/g. The admixture of finely divided inorganic solids preferablytakes place after the surface modification throughcrosslinking/complexing, but may also be carried out before or duringthese surface modifications. The surface-postcrosslinked material isgenerally heat treated.

Heat treatment jacket temperature: 120-180° C., preferably 140-160° C.,especially 150° C.; heat treatment residence time has to be conformed tothe temperature, higher temperatures involving shorter residence timesand longer residence times giving rise to more pronouncedpostcrosslinking. Typical values are 150-10 minutes.

AUL and CRC can be optimized by controlling the postcrosslinking time.

Copolymers of C₂-C₈ olefins or styrenes with anhydrides.

Copolymers of C₂-C₈ olefins or styrenes with anhydrides are known andcommercially obtainable. Their preparation has been exhaustivelydescribed, for example in US 5066742 and US 5026784, whose method ofmaking is hereby incorporated into the present invention by reference.

Properties of Polymer Mixtures According to the Present Invention

The hydrogel-forming polymers capable of absorbing aqueous fluids have aparticle size distribution which is generally in the range from 10 μm toabout 1 000 μm, preferably in the range from about 100 μm to about 850μm and especially in the range from 150 μm to about 700 μm. The sizewindow mentioned preferably includes more than 80% by weight andespecially more than 90% by weight of the particles.

The odor-binding copolymers of C₂-C₈ olefins or styrenes with anhydrideshave a particle size distribution which is generally in the range from10 μm to about 600 μm, preferably in the range from about 100 μm toabout 400 μm and especially in the range from 150 μm to about 300 μm.The size window mentioned preferably includes more than 80% by weightand especially more than 90% by weight of the particles.

The C₂-C₈ olefin-anhydride, especially maleic anhydride, copolymerfibers capable of absorbing aqueous fluids are preferably obtained bythe method of U.S. Pat. No. 5,026,784 example 1 column 8 line 24 andhave the properties described there (degree of neutralization 55%,diameter of noncrosslinked fiber: 2-3 denier).

The polymer mixtures comprise improved odor control properties as wellas high ultimate absorption capacity, high gel strength and permeabilityand also high retention. Owing to the presence of copolymers of C₂-C₈olefins or styrenes with anhydrides, the products of the presentinvention have antimicrobial properties, thereby providing an odorcontrol system which obviates the addition of further odor-inhibitingsubstances or odor-masking materials.

The addition of partially neutralized copolymers of C₂-C₈ olefins orstyrenes with anhydrides in fiber form provides higher acquisition ratesand also higher retention values than is the case with the granularpolymer mixture of the present invention.

In contrast to the prior art, where an added odor control unit to thesuperabsorbent polymer leads to a decrease in the absorptiveperformance, the polymer mixture of the present invention has no adverseeffect on the absorption profile. Moreover, the products of theinvention permit substantially less costly manufacture, since there isno need for binders or other aids for binding an odor control unit tohydrogel-forming polymers.

The high absorptive performance and an unchanged absorptive profile onthe part of the hydrogel-forming polymers used permits longer wear timeswhen the products of the present invention are used in a hygienearticle. Skin sensitization and irritation is completely avoided andeliminated by a constant pH medium.

Deployment and Use of the Polymer Mixture

The present invention further provides for the use of the abovementionedpolymer mixtures in hygiene articles comprising

-   -   (A) a liquid pervious topsheet    -   (B) a liquid impervious backsheet    -   (C) a core positioned between (A) and (B) and comprising 10-100%        by weight of the polymer mixture according to the invention        -   0-90% by weight of hydrophilic fiber material preferably            20-100% by weight of the polymer mixture according to the            invention, 0-80% by weight of the hydrophilic fiber material        -   more preferably 30-100% by weight of the polymer mixture            according to the invention, 0-70% by weight of the            hydrophilic fiber material        -   even more preferably 40-100% by weight of the polymer            mixture according to the invention, 0-60% by weight of the            hydrophilic fiber material        -   much more preferably 50-100% by weight of the polymer            mixture according to the invention, 0-50% by weight of the            hydrophilic fiber material        -   particularly preferably 60-100% by weight of the polymer            mixture according to the invention, 0-40% by weight of the            hydrophilic fiber material        -   especially preferably 70-100% by weight of the polymer            mixture according to the invention, 0-30% by weight of the            hydrophilic fiber material        -   extremely preferably 80-100% by weight of the polymer            mixture according to the invention, 0-20% by weight of the            hydrophilic fiber material        -   most preferably 90-100% by weight of the polymer mixture            according to the invention, 0-10% by weight of the            hydrophilic fiber material    -   (D) optionally a tissue layer positioned directly above and        below said core (C) and    -   (E) optionally an acquisition layer positioned between (A) and        (C).

The hydrophilic fiber material can be wholly or partly replaced by fibermaterial composed of copolymers of C₂-C₈ olefins or styrenes withanhydrides. The preferred percentages are to be understood so that inthe case of 10-100% by weight 11, 12, 13, 14, 15, 16, 17, 18, 19 up toin each case 100% by weight of polymer mixture according to theinvention and all in between % ages (for example 12.2%) are possible andcorrespondingly hydrophilic fiber material from 0 to respectively 89,88, 87, 86, 85, 83, 82, 81% by weight and in between percentages (forexample 87.8%) are possible. If further materials are present in thecore, the percentages of polymer and fiber decrease accordingly. Thesame applies to the preferred ranges, for example in the case ofextremely preferably 81, 82, 83, 84, 85, 86, 87, 88, 89% by weight canbe present for the polymer mixture according to the invention andcorrespondingly 19, 18, 17, 16, 15, 14, 13, 12, 11% by weight of thefiber material. So the preferred range contains 20, 21, 22, 23, 24, 25,26, 27, 28, 29 to 100% by weight of the polymer mixture according to theinvention, the more preferred range 30, 31, 32, 33, 34, 35, 36, 37, 38,39 to 100% by weight of the polymer mixture according to the invention,the even more preferred range 40, 41, 42, 43, 44, 45, 46, 47, 48, 49 to100% by weight of polymer mixture according to the invention, the muchmore preferred range 50, 51, 52, 53, 54, 55, 56, 57, 58, 59 to 100% byweight of polymer mixture according to the invention, the particularlypreferred range 60, 61, 62, 63, 64, 65, 66, 67, 68, 69 to 100% by weightof polymer mixture according to the invention, the especially preferredrange 70, 71, 71, 72, 73, 74, 75, 76, 77, 78, 79 to 100% by weight ofpolymer mixture according to the invention and the most preferred range90, 91, 92, 93, 94, 95, 96, 97, 98, 99 or 100% by weight of polymermixture according to the invention.

Hygiene articles for the purposes of the present invention include notonly incontinence pads and incontinence briefs for adults but alsodiapers for infants.

The liquid pervious topsheet (A) is the layer which is in direct contactwith the skin of the wearer. Its material comprises customary syntheticor manufactured fibers or films of polyesters, polyolefins, rayon ornatural fibers such as cotton. In the case of non-woven materials thefibers are generally joined together by binders such as polyacrylates.Preferred materials are polyesters, rayon or blends thereof,polyethylene and polypropylene. Examples of liquid pervious layers aredescribed in WO 99/57355 A1, EP 102 388 3 A2.

The liquid impervious layer (B) is generally a sheet of polyethylene orpolypropylene.

The core (C) includes not only the polymer mixture of the invention butalso hydrophilic fiber material. By hydrophilic is meant that aqueousfluids spread quickly over the fiber. The fiber material is usually acellulose, modified cellulose, rayon, polyester such as polyethyleneterephthlate. Particular preference is given to cellulose fibers such aspulp. The fibers generally have a diameter of 1-200 μm, and preferably10-100 μm, and also have a minimum length of 1 mm.

Diaper construction and shape is common knowledge and described forexample in WO 95/26209 page 66 line 34 to page 69 line 11, DE 196 04 601A1, EP-A-0 316 518 and EP-A-0 202 127. Diapers and other hygienearticles are generally also described in WO 00/65084, especially atpages 6-15, WO 00/65348, especially at pages 4-17, WO 00/35502,especially pages 3-9, DE 19737434, WO 98/8439. Hygiene articles forfeminine hygiene are described in the following references. Theinventive polymer mixtures capable of absorbing aqueous fluids can beused there. Femcare references: WO 95/24173: Absorption Article forControlling Odour, WO 91/11977: Body Fluid Odour Control, EP 389023:Absorbent Sanitary Articles, WO 94/25077: Odour Control Material, WO97/01317: Absorbent Hygienic Article, WO 99/18905, EP 834297, U.S. Pat.No. 5,762,644, U.S. Pat. No. 5,895,381, WO 98/57609, WO 2000/065083, WO2000/069485, WO 2000/069484, WO 2000/069481, U.S. Pat. No. 6,123,693, EP1104666, WO 2001/024755, WO 2001/000115, EP 105373, WO 2001/041692, EP1074233. Tampons are described in the following references: WO 98/48753,WO 98/41179, WO 97/09022, WO 98/46182, WO 98/46181, WO 2001/043679, WO2001/043680, WO 2000/061052, EP 1108408, WO 2001/033962, DE 200020662,WO 2001/001910, WO 2001/001908, WO 2001/001909, WO 2001/001906, WO2001/001905, WO 2001/24729. Incontinence articles are described in thefollowing references: Disposable Absorbent Article for IncontinentIndividuals: EP 311344 description pages 3-9; Disposable AbsorbentArticle: EP 850623; Absorbent Article: WO 95/26207; Absorbent Article:EP 894502; Dry Laid Fibrous Structure: EP 850 616; WO 98/22063; WO97/49365; EP 903134; EP 887060; EP 887059; EP 887058; EP 887057; EP887056; EP 931530; WO 99/25284; WO 98/48753. Femcare and incontinencearticles are described in the following references: Catamenial Device:WO 93/22998 description pages 26-33; Absorbent Members for Body Fluids:WO 95/26209 description pages 36-69; Disposable Absorbent Article: WO98/20916 description pages 13-24; Improved Composite AbsorbentStructures: EP 306262 description pages 3-14; Body Waste AbsorbentArticle: WO 99/45973. These references and the references therein arehereby expressly incorporated in the disclosure of the presentinvention.

Alternatively, the core (C) can also be composed of layers of thecomponents (i) and (ii). It is in principle possible for multilayerconstructions of 3 (eg layer of component (i)/layer of component(ii)/layer of component (i) or layer of component (ii)/layer ofcomponent (i)/layer of component (ii)), 4, 5 or more layers to bepresent, but preference is given to constructions having two layers, inwhich case not only the layer of component (i) but also the layer ofcomponent (ii) can be closer to the body. Other possibilities are layerconstructions composed of layers comprising polymer mixtures accordingto the present invention and layers of individual components (i) and/or(ii).

The polymer mixtures according to the invention are very useful asabsorbents for water and aqueous fluids, so that they may be used withadvantage as a water retainer in market gardening, as a filter aid andparticularly as an absorbent component in hygiene articles such asdiapers, tampons or sanitary napkins.

Incorporation and Fixation of the Highly Swellable Polymer MixturesAccording to the Invention

In addition to the above-described polymer mixture of hydrogel-formingpolymers and copolymers of C₂-C₈ olefins or styrenes with anhydrides,the absorbent composition of the present invention may includeconstructions which include the polymer mixture or to which they arefixed. Any construction is suitable that is capable of accommodating thepolymer mixture and of being integrated into the absorption layer. Amultiplicity of such compositions is already known and described indetail in the literature. A construction for installing the polymermixture can be for example a fiber matrix consisting of a cellulosefiber mixture (air-laid web, wet laid web) or synthetic polymer fibers(meltblown web, spunbonded web) or else of a fiber blend of cellulosefibers and synthetic fibers. Possible fiber materials are detailed inthe chapter which follows. The air-laid web process is described forexample in WO 98/28 478. Furthermore, open-celled foams or the like maybe used to install the polymer mixture according to the invention.

Alternatively, such a construction can be the result of fusing twoindividual layers to form one or better a multiplicity of chambers whichcontain the polymer mixture. Such a chamber system is described indetail in EP 0 615 736 A1 page 7 lines 26 et seq.

In this case, at least one of the two layers should be water pervious.The second layer may either be water pervious or water impervious. Thelayer material used may be tissues or other fabric, closed oropen-celled foams, perforated films, elastomers or fabrics composed offiber material. When the absorbent composition consists of aconstruction of layers, the layer material should have a pore structurewhose pore dimensions are small enough to retain the particles of thepolymer mixture. The above examples on the construction of the absorbentcomposition also include laminates composed of at least two layersbetween which the polymer mixture is installed and fixed.

Generally it is possible to fix the particles of the polymer mixturewithin the absorbent core to improve dry and wet integrity. Dry and wetintegrity describes the ability to install highly swellable hydrogelsinto the absorbent composition in such a way that they withstandexternal forces not only in the wet but also in the dry state and highlyswellable polymer does not dislocate or spill out. The forces referredto are especially mechanical stresses as occur in the course of movingabout while wearing the hygiene article or else the weight pressure onthe hygiene article in the case of incontinence especially. As tofixation, one skilled in the art knows a multiplicity of possibilities.Examples such as fixation by heat treatment, addition of adhesives,thermoplastics, binder materials are noted in WO 95/26 209 page 37 line36 to page 41 line 14. The cited passage is thus part of this invention.Methods for enhancing wet strength are also to be found in WO 2000/36216A1.

Furthermore, the absorbent composition may comprise a base material, forexample a polymer film on which the polymer mixture is fixed. The fixingmay be effected not only on one side but also on both sides. The basematerial can be water pervious or water impervious.

The above constructions of the absorbent composition incorporateparticles of the polymer mixture at a weight fraction of from 10 to 100%by weight, preferably 20-100% by weight, more preferably 30-100% byweight, even more preferably 40-100% by weight, much more preferably50-100% by weight, particularly preferably 60-100% by weight, especiallypreferably 70-100% by weight, extremely preferably 80-100% by weight andmost preferably 90-100% by weight, based on the total weight of theconstruction and of the polymer mixture.

Fiber Materials of the Absorbent Composition

The structure of the present absorbent composition according to theinvention may be based on various fiber materials, which are used as afiber network or matrices. The present invention includes not onlyfibers of natural origin (modified or unmodified) but also syntheticfibers.

A detailed overview of examples of fibers which can be used in thepresent invention is given in WO 95/26 209 page 28 line 9 to page 36line 8. The cited passage is thus part of this invention.

Examples of cellulose fibers include cellulose fibers which arecustomarily used in absorption products, such as fluff pulp andcellulose of the cotton type. The materials (soft- or hardwoods),production processes such as chemical pulp, semichemical pulp,chemothermo-mechanical pulp (CTMP) and bleaching processes are notparticularly restricted. For instance, natural cellulose fibers such ascotton, flax, silk, wool, jute, ethylcellulose and cellulose acetate areused.

Suitable synthetic fibers are produced from polyvinyl chloride,polyvinyl fluoride, polytetrafluoroethylene, polyvinylidene chloride,polyacrylic compounds such as ORLON®, polyvinyl acetate, polyethyl vinylacetate, soluble or insoluble polyvinyl alcohol. Examples of syntheticfibers include thermoplastic polyolefin fibers, such as polyethylenefibers (PULPEX®), polypropylene fibers and polyethylene-polypropylenebicomponent fibers, polyester fibers, such as polyethylene terephthalatefibers (DACRON® or KODEL®), copolyesters, polyvinyl acetate, polyethylvinyl acetate, polyvinyl chloride, polyvinylidene chloride,polyacrylics, polyamides, copolyamides, polystyrene and copolymers ofthe aforementioned polymers and also bicomponent fibers composed ofpolyethylene terephthalate-polyethylene-isophthalate copolymer,polyethyl vinyl acetate/polypropylene, polyethylene/polyester,polypropylene/polyester, copolyester/polyester, polyamide fibers(nylon), polyurethane fibers, polystyrene fibers and polyacrylonitrilefibers. Preference is given to polyolefin fibers, polyester fibers andtheir bicomponent fibers. Preference is further given to thermallyadhesive bicomponent fibers composed of polyolefin of the core-sheathtype and side-by-side type on account of their excellent dimensionalstability following fluid absorption.

The synthetic fibers mentioned are preferably used in combination withthermoplastic fibers. In the course of the heat treatment, the lattermigrate to some extent into the matrix of the fiber material present andso constitute bond sites and renewed stiffening elements on cooling.Additionally the addition of thermoplastic fibers means that there is anincrease in the present pore dimensions after the heat treatment hastaken place. This makes it possible, by continuous addition ofthermoplastic fibers during the formation of the absorbent core, tocontinuously increase the fraction of thermoplastic fibers in thedirection of the topsheet, which results in a similarly continuousincrease in the pore sizes. Thermoplastic fibers can be formed from amultiplicity of thermoplastic polymers which have a melting point ofless than 190° C., preferably in the range from 75° C. to 175° C. Thesetemperatures are too low for damage to the cellulose fibers to belikely.

Lengths and diameters of the above-described synthetic fibers are notparticularly restricted, and generally any fiber from 1 to 200 mm inlength and from 0.1 to 100 denier (gram per 9 000 meters) in diametermay preferably be used. Preferred thermoplastic fibers are from 3 to 50mm in length, particularly preferred thermoplastic fibers are from 6 to12 mm in length. The preferred diameter for the thermoplastic fiber isin the range from 1.4 to 10 decitex, and the range from 1.7 to 3.3decitex (gram per 10 000 meters) is particularly preferred. The form ofthe fiber may vary; examples include woven types, narrow cylindricaltypes, cut/chopped yarn types, staple fiber types and continuousfilament fiber types.

The fibers in the absorbent composition of the invention can behydrophilic, hydrophobic or a combination thereof. According to thedefinition of Robert F. Gould in the 1964 American Chemical Societypublication “Contact angle, wettability and adhesion”, a fiber isreferred to as hydrophilic when the contact angle between the liquid andthe fiber (or the fiber surface) is less than 90° or when the liquidtends to spread spontaneously on the same surface. The two processes aregenerally coexistent. Conversely, a fiber is termed hydrophobic when acontact angle of greater than 90° is formed and no spreading isobserved.

Preference is given to using hydrophilic fiber material. Particularpreference is given to using fiber material which is weakly hydrophilicon the body side and most hydrophilic in the region surrounding thepolymer mixture. In the manufacturing process, layers having differenthydrophilicities are used to create a gradient which channels impingingfluid to the hydrogel, where it is ultimately absorbed.

Suitable hydrophilic fibers for use in the absorbent composition of theinvention include for example cellulose fibers, modified cellulosefibers, rayon, polyester fibers, for example polyethylene terephthalate(DACRON®), and hydrophilic nylon (HYDROFIL®). Suitable hydrophilicfibers may also be obtained by hydrophilicizing hydrophobic fibers, forexample the treatment of thermoplastic fibers obtained from polyolefins(e.g. polyethylene or polypropylene, polyamides, polystyrenes,polyurethanes, etc.) with surfactants or silica. However, for costreasons and ease of availability, cellulosic fibers are preferred.

The polymer mixture is embedded in the fibrous material described. Thiscan be done in various ways, for example by using the polymer materialand the fibers together to create an absorbent layer in the form of amatrix, or by incorporating the polymer particle mixture in layers offiber mixture, where they are ultimately fixed, whether by means ofadhesive or by lamination of the layers.

The fluid-acquiring and -distributing fiber matrix may comprisesynthetic fiber or cellulosic fiber or a mixture of synthetic fiber andcellulosic fiber, in which case the mixing ratio may vary from (100 to0) synthetic fiber: (0 to 100) cellulosic fiber. The cellulosic fibersused may additionally have been chemically stiffened to increase thedimensional stability of the hygiene article.

The chemical stiffening of cellulosic fibers may be provided indifferent ways. A first way of providing fiber stiffening is by addingsuitable coatings to the fiber material. Such additives include forexample polyamide-epichlorohydrin coatings (Kymene® 557 H, Hercoles,Inc. Wilmington, Del.), polyacrylamide coatings (described in U.S. Pat.No. 3,556,932 or as the Parez® 631 NC commercial product from AmericanCyanamid Co., Stamford, Conn.), melamine-formaldehyde coatings andpolyethyleneimine coatings.

Cellulosic fibers may also be chemically stiffened by chemical reaction.For instance, suitable crosslinker substances may be added to effectcrosslinking taking place within the fiber. Suitable crosslinkersubstances are typical substances used for crosslinking monomersincluding but not limited to C₂-C₈-dialdehydes, C₂-C₈-monoaldehydeshaving acid functionality and in particular C₂-C₉-polycarboxylic acids.Specific substances from this series are for example glutaraldehyde,glyoxal, glyoxylic acid, formaldehyde and citric acid. These substancesreact with at least 2 hydroxyl groups within any one cellulose chain orbetween two adjacent cellulose chains within any one cellulose fiber.The crosslinking causes a stiffening of the fibers, to which greaterdimensional stability is imparted as a result of this treatment. Inaddition to their hydrophilic character, these fibers exhibit uniformcombinations of stiffening and elasticity. This physical property makesit possible to retain the capillary structure even under simultaneouscontact with fluid and compressive forces and to prevent prematurecollapse.

Chemically crosslinked cellulose fibers are known and described in WO91/11162, U.S. Pat. No. 3,224,926, U.S. Pat. No. 3,440,135, U.S. Pat.No. 3,932,209, U.S. Pat. No. 4,035,147, U.S. Pat. No. 4,822,453, U.S.Pat. No. 4,888,093, U.S. Pat. No. 4,898,642 and U.S. Pat. No. 5,137,537.The chemical crosslinking imparts stiffening to the fiber material,which is ultimately reflected in improved dimensional stability for thehygiene article as a whole. The individual layers are joined together bymethods known to one skilled in the art, for example intermelting byheat treatment, addition of hot-melt adhesives, latex binders, etc.

Methods of Making the Absorbent Composition

The absorbent composition is composed of constructions which include thepolymer mixture and the polymer mixture which is resent in saidconstructions or fixed thereto.

Examples of processes to obtain an absorbent composition comprising forexample a base material to which particles of the polymer mixture arefixed on one or both sides are known and included by the invention butnot limited thereto.

Examples of processes to obtain an absorbent composition comprising forexample polymer mixture (c) embedded in a fiber material blend ofsynthetic fibers (a) and cellulosic fibers (b), the blend ratio varyingfrom (100 to 0) synthetic fiber: (0 to 100) cellulosic fiber, include(1) a process where (a), (b) and (c) are mixed together at one and thesame time, (2) a process where a mixture of (a) and (b) is mixed into(c), (3) a process where a mixture of (b) and (c) is mixed with (a), (4)a process where a mixture of (a) and (c) is mixed into (b), (5) aprocess where (b) and (c) are mixed and (a) is continuously metered in,(6) a process where (a) and (c) are mixed and (b) is continuouslymetered in, and (7) a process where (b) and (c) are mixed separatelyinto (a). Of these examples, processes (1) and (5) are preferred. Theapparatus used in this process is not particularly restricted and anycustomary apparatus known to one skilled in the art can be used.

The absorbent composition obtained in this way can optionally besubjected to a heat treatment, so that an absorption layer havingexcellent dimensional stability in the moist state is obtained. The heattreatment process is not particularly restricted. Examples include heattreatment by feeding hot air or infrared irradiation. The temperature ofthe heat treatment is in the range from 60° C. to 230° C., preferablyfrom 100° C. to 200° C., particularly preferably from 100° C. to 180° C.

The duration of the heat treatment depends on the type of syntheticfiber, its amount and the hygiene article production rate. Generally theduration of the heat treatment is in the range from 0.5 second to 3minutes, preferably from 1 second to 1 minute.

The absorbent composition is generally provided for example with aliquid-pervious topsheet and a liquid-impervious backsheet. Furthermore,leg cuffs and adhesive tabs are attached to finalize the hygienearticle. The materials and types of pervious topsheet and imperviousbacksheet and of the leg cuffs and adhesive tabs are known to oneskilled in the art and are not particularly restricted. Examples thereofmay be found in WO 95/26 209.

Test Methods

a) Centrifuge Retention Capacity (CRC)

This method measures the free swellability of the polymer mixture in ateabag. 0.2000±0.0050 g of the polymer mixture of the invention, e.g.dry polymer mixture, consisting of 0.18 g of hydrogel-forming polymer(particle size fraction 106-850 μm) and 0.02 g of copolymers of C₂-C₈olefins or styrenes with anhydrides (particle size fraction 100-400 μm)are weighed into a teabag 60×85 mm in size which is subsequently sealed.The teabag is placed for 30 minutes in an excess of 0.9% by weightsodium chloride solution (at least 0.83 l of sodium chloride solution/1g of polymer powder). The teabag is then centrifuged for 3 minutes at250 g. The amount of liquid is determined by weighing back thecentrifuged teabag.

To determine the CRC in the comparative tests the hydrogel-formingpolymer was used alone in place of the polymer mixture (0.2000±0.0050g).

b) Absorbency Under Load (AUL) (0.7 psi)

The measuring cell for determining AUL 0.7 psi is a Plexiglass cylinder60 mm in internal diameter and 50 mm in height. Adhesively attached toits underside is a stainless steel sieve bottom having a mesh size of 36μm. The measuring cell further includes a plastic plate having adiameter of 59 mm and a weight which can be placed in the measuring celltogether with the plastic plate. The plastic plate and the weighttogether weigh 1 345 g. AUL 0.7 psi is determined by determining theweight of the empty Plexiglass cylinder and of the plastic plate andrecording it as W₀. 0.900±0.005 g of a polymer mixture of the invention,eg polymer mixture consisting of 0.81 g of hydrogel-forming polymer(particle size fraction 106-850 μm) and 0.09 g of copolymers of C₂-C₈olefins or styrenes with anhydrides (particle size fraction 100-400 μm)are then weighed into the Plexiglass cylinder and distributed veryuniformly over the stainless steel sieve bottom. The plastic plate isthen carefully placed in the Plexiglass cylinder, the entire unit isweighed and the weight is recorded as W_(a). The weight is then placedon the plastic plate in the Plexiglass cylinder. A ceramic filter plate120 mm in diameter and 0 in porosity is then placed in the middle of thePetri dish 200 mm in diameter and 30 mm in height and sufficient 0.9% byweight sodium chloride solution is introduced for the surface of theliquid to be level with the filter plate surface without the surface ofthe filter plate being wetted. A round filter paper 90 mm in diameterand <20 μm in pore size (S&S 589 Schwarzband from Schleicher & Schüll)is subsequently placed on the ceramic plate. The Plexiglass cylindercontaining the polymer mixture is then placed with plastic plate andweight on top of the filter paper and left there for 60 minutes. At theend of this period, the complete unit is removed from the filter paperand the Petri dish and subsequently the weight is removed from thePlexiglass cylinder. The Plexiglass cylinder containing swollen hydrogelmixture is weighed together with the plastic plate and the weightrecorded as W_(b).

AUL was calculated by the following equation:AUL 0.7 psi [g/g]=[W _(b) −W _(a) ]/[W _(a) −W ₀]

AUL 0.5 psi is measured using a correspondingly lighter weight on theplastic plate.

To determine AUL 0.7 psi and 0.5 psi, respectively, in the comparativetests the hydrogel-forming polymer was used alone in place of thepolymer mixture (0.9000±0.005 g).

c) Saline Flow Conductivity (SFC)

The test method for determining SFC is described in U.S. Pat. No. 5 599335.

d) pH Measurement of Hydrogel-Forming Polymers

100 ml of 0.9% by weight NaCl solution are magnetically stirred atmoderate speed in a 150 ml beaker without air being drawn into thesolution. This solution is admixed with 0.5±0.001 g of the polymer to bemeasured and stirred for 10 minutes. After 10 minutes, the pH of thesolution is measured with a pH glass electrode, the value not being readoff until it is stable, but at the earliest after 1 minute.

e) Measuring the Buffering Capacity of Hydrogel-Forming Polymers

To determine the buffering capacity of the hydrogel-forming polymers,0.5±0.001 g of hydrogel-forming polymer or polymer mixture is placed in100 ml of 0.9% by weight NaCl solution in a 150 ml glass beaker andmagnetically stirred at moderate speed, so that the stirring does notdraw any air into the solution.

After 10 minutes, the pH of the solution is measured for the first timewith a pH glass electrode, the value not being read off until it isstable, but at the earliest after 1 minute. Subsequently, 0.1 molar NaOHsolution is then added by 0.05 ml of 0.1 molar NaOH solution beingmetered in every 5 minutes with continued stirring. The pH of themixture was continually checked in the course of the addition. Thebuffering capacity was determined from the pH prior to the addition ofthe 0.1 molar NaOH solution and from the pH after 6 hours.

EXAMPLES

The polymer mixtures obtained in the inventive examples aredistinguished from the polymers obtained in the comparative examples bya combination of absorption quantity and swell rate and exhibit a highfluid permeability and also improved odor control properties. They aretherefore very useful as absorbents for water and aqueous fluids,especially body fluids, for example urine or blood, for example inhygiene articles such as for example infant and adult diapers, sanitarynapkins, tampons and the like.

The examples hereinbelow illustrate the invention.

Comparative Example 1

a) In a 40 l plastic bucket, 6.9 kg of glacial acrylic acid are dilutedwith 20 kg of deionized water. 33 g of pentaerythritol triallyl etherare added to this solution with stirring, and the sealed bucket isinertized by passing nitrogen through it. The polymerization is theninitiated by adding 0.4 g of hydrogen peroxide dissolved in 40 ml ofdeionized water and 0.2 g of ascorbic acid dissolved in 40 ml ofdeionized water. After the reaction has ended, the gel is mechanicallycomminuted and mixed with sufficient aqueous sodium hydroxide solutionfor a degree of neutralization of 75 mol %, based on acrylic acid used.The neutralized gel is then dried on a can dryer, ground with a pin milland finally screened off at 150-850 μm.

b) The base polymer prepared under a) was sprayed with 2.9% by weight ofcrosslinker solution composed of 49.56 parts by weight of1,2-propanediol, 49.56 parts by weight of deionized water and 0.88 partby weight of monoethylene glycol diglycidyl ester (EDGE) in a Lödigelaboratory mixer, the percentages being based on base polymer. The moistproduct was then transferred into a second preheated Lödige laboratorymixer and annealed at 140° C. for 60 minutes. The dried product wascooled down to room temperature and screened off at 850 μm.

Comparative Example 2

a) In a 40 1 plastic bucket, 6.9 kg of glacial acrylic acid are dilutedwith 20 kg of deionized water. 33 g of pentaerythritol triallyl etherare added to this solution with stirring, and the sealed bucket isinertized by passing nitrogen through it. The polymerization is theninitiated by adding 0.4 g of hydrogen peroxide dissolved in 40 ml ofdeionized water and 0.2 g of ascorbic acid dissolved in 40 ml ofdeionized water. After the reaction has ended, the gel is mechanicallycomminuted and mixed with sufficient aqueous sodium hydroxide solutionfor a degree of neutralization of 75 mol %, based on acrylic acid used.The neutralized gel is then dried on a can dryer, ground with a pin milland finally screened off at 150-850 μm.

b) The base polymer prepared under a) was sprayed with 3.75% by weightof crosslinker solution composed of 33.3 parts by weight of1,2-propanediol, 63.5 parts by weight of deionized water and 3.2 partsby weight of EDGE and also with 0.12 part by weight of a 27% aqueousaluminum sulfate solution in a Lödige laboratory mixer, the percentagesbeing based on base polymer. Crosslinker solution and aluminum sulfatesolution are sprayed separately but simultaneously from 2 nozzles. Themoist product was then transferred into a second preheated Lödigelaboratory mixer and annealed at 140° C. for 60 minutes. The driedproduct was cooled down to room temperature and screened off at 850 μm.

Comparative Example 3

A 10 l capacity polyethylene vessel thoroughly insulated with foamedplastic material is charged with 3 928 g of completely ion-free water,625 g of sodium bicarbonate are suspended in the water and 2 000 g ofacrylic acid are added with stirring so that there is no over-foamingdue to ensuing CO₂ evolution. This is followed by the addition, insuccession, of an emulsion of 1.3 g of sorbitan monococoate in 100 g ofcompletely ion-free water and 8.1 g of allyl methacrylate, and thesolution is further inertized by passing nitrogen into it. This isfollowed by the addition of the initiator system, consisting of 1.66 gof 2,2′-azobisamidinopropane dihydrochloride (dissolved in 20 g ofcompletely ion-free water), 3.33 g of potassium peroxodisulfate(dissolved in 150 g of completely ion-free water) and also 0.3 g ofascorbic acid (dissolved in 25 g of completely ion-free water) insuccession with stirring. The reaction solution is then left to standwithout stirring. The polymerization which ensues, and in the course ofwhich the temperature rises to about 90° C., produces a solid gel. Thissolid gel is mechanically comminuted using a meat grinder, dried on VAstainless steel wire mesh in a circulating air drying cabinet at 160°C., then ground and screened.

Comparative Example 4

TYLOSE VS 3790, a superabsorbent from CASSELLA AG of Frankfurt/Main,characterized by a pH of 5-5.5, prepared similarly to example 7 of EP 0316 792 B1, was admixed on a 20 g scale in a WARING blender (modifiedattachment for kitchen processor) with a surface-postcrosslinkingsolution (spray from 2 ml syringe), consisting of 2.3% of water/1.2% of1,2-propanediol/0.2% of ethylene glycol diglycidyl ether (eachpercentage being based on polymer) and heat treated in a circulating airdrying cabinet at 140° C. for one hour.

Comparative Example 5

A WERNER & PFLEIDERER laboratory kneader having a working capacity of 21 is evacuated to 980 mbar absolute by means of a vacuum pump and apreviously separately prepared monomer solution which has been cooled toabout 25° C. and inertized by passing nitrogen into it is sucked intothe kneader. The monomer solution has the following composition: 825.5 gof completely ion-free water, 431 g of acrylic acid, 120.68 g of 50%NaOH, 0.86 g of polyethylene glycol 400 diacrylate (SARTOMER® 344 fromCRAY VALLEY). To improve the inertization, the kneader is evacuated andsubsequently refilled with nitrogen. This operation is repeated 3 times.A solution of 1.2 g of sodium persulfate (dissolved in 6.8 g ofcompletely ion-free water) is then sucked in, followed after a further30 seconds by a further solution consisting of 0.024 g of ascorbic aciddissolved in 4.8 g of completely ion-free water. After a nitrogen purgea preheated jacket heating circuit on bypass at 75° C. is switched overto the kneader jacket and the stirrer speed increased to 96 rpm. Afterensuing polymerization and the attainment of T_(max), the jacket heatingcircuit is reswitched back to bypass, and the batch is supplementarilypolymerized for 15 minutes without heating/cooling, subsequently cooledand discharged. The resultant gel particles are dried at 160° C. on wiremesh bottomed trays in a circulating air drying cabinet and then groundand screened.

1 200 g of the thus obtained product of the particle size distribution105-850 μm were sprayed with a homogeneous solution consisting of 17.58g of water, 9.96 g of 1,2-propanediol, 1.2 g of ethylene glycoldiglycidyl ether and 3.36 g of 26.8% aqueous aluminum sulfate solutionin a powder mixing assembly (Lödige mixer) and transferred into asecond, preheated Lödige mixer. The heat treatment was carried out for aperiod of 70 minutes under constant conditions of 150° C. jackettemperature and stirrer speed 60 rpm. The mixer was emptied, the productwas cooled to room temperature and screened off at 850 μm.

Inventive Example 1

5 parts of powder of the 1/1 i-butylene/maleic anhydride copolymer fromKuraray Isoprene Chemical Company, Ltd., (Tokyo, Japan, trade name:ISOBAM®) and 95 parts from comparative example 4 are mixed in alaboratory tumble mixer for 60 minutes until homogeneous.

Inventive Example 2

10 parts of powder of the 1/1 i-butylene/maleic anhydride copolymer fromKuraray Isoprene Chemical Company, Ltd., (Tokyo, Japan, trade name:ISOBAM®) and 90 parts from comparative example 4 are mixed in alaboratory tumble mixer for 60 minutes until homogeneous.

Inventive Example 3

5 parts of powder of the 1/1 i-butylene/maleic anhydride copolymer fromKuraray Isoprene Chemical Company, Ltd., (Tokyo, Japan, trade name:ISOBAM®) and 95 parts from comparative example 2 are mixed in alaboratory tumble mixer for 60 minutes until homogeneous.

Inventive Example 4

10 parts of powder of the 1/1 i-butylene/maleic anhydride copolymer fromKuraray Isoprene Chemical Company, Ltd., (Tokyo, Japan, trade name:ISOBAM®) and 90 parts from comparative example 2 are mixed in alaboratory tumble mixer for 60 minutes until homogeneous.

Inventive Example 5

50 parts of powder of the i-butylene/maleic anhydride copolymer fibersprepared as per the method of U.S. Pat. No. 5,026,784 Example 1 column 8line 24 and 50 parts from comparative example 1 are mixed in alaboratory tumble mixer for 60 minutes until homogeneous.

The absorptive performance data of the examples are discernible fromtable 1, while the odor control properties are approximated in table 2using the pH as measured after 6 hours (buffering capacity). TABLE 1 AULSFC CRC 0.5 psi AUL 0.7 psi Example pH ×10⁻⁷ cm³s/g g/g g/g g/gComparative 1 5.95 5 33.4 29.4 22.8 Comparative 2 6.1 15 29 26 23Comparative 3 4.5 3 23 11 7 Comparative 4 5.4 ≦1 42.0 6.0 Comparative 54.47 13.8 20.7 18.1 Inventive 1 5.4 9 42.0 6.1 Inventive 2 5.4 14 40.85.9 Inventive 3 6.1 18 28.7 25.5 23 Inventive 4 6.1 24 28.9 25.9 22.3Inventive 5 6.4 ≦1 41.8 28.7 20.2

TABLE 2 pH after 6 hours (buffering Example pH capacity) Comparative 15.95 6.6 Comparative 2 6.1 7.1 Comparative 3 4.5 4.7 Comparative 5 4.474.6 Inventive 1 5.4 5.5 Inventive 2 5.4 5.4 Inventive 3 6.1 6.2Inventive 4 6.1 5.9

The results show that the inventive examples provide distinctly improvedodor control coupled with substantially the same absorptive performance.The distinctly improved odor control was demonstrated in the test by theenormous buffering capacity on titration with 0.1 molar NaOH solution.Comparative example 3 and comparative example 5 admittedly likewise givegood odor control at a lower pH than inventive examples 1 and 2, but theperformance with regard to CRC in particular is distinctly worse.

Comparative examples 1 and 2 are each a hydrogel-forming material fromthe prior art, which has been optimized especially with regard toAbsorbency Under Load at pH 5.95 and pH 6.1 respectively, whereas theodor control properties (buffering capacity) are moderate. However,mixing this material with copolymer fibers (in a ratio of 1:1 incomparative example 1) gives increased retention values.

1-11. (canceled)
 12. A polymer mixture including components (i) ahydrogel-forming polymer capable of absorbing aqueous fluids andprepared by polymerizing an olefinically unsaturated carboxylic acid ora derivative thereof, and (ii) a copolymer of a C₂-C₈ olefin or styrenewith an anhydride in a molar ratio between the C₂-C₈ olefin or styreneand the anhydride in a range from 3:1 to 1:3.
 13. The polymer mixture ofclaim 1 wherein component (i) is granular or fibrous, and component (ii)is independently granular or fibrous, and optionally component (ii) isadditionally fibrous or granular.
 14. The polymer mixture of claim 1wherein component (ii) is sprayed onto component (i) as a polymer or asa monomer mixture with subsequent polymerization.
 15. The polymermixture of claim 1 wherein component (i) comprises a polyacrylate. 16.The polymer mixture of claim 1 wherein component (ii) is granular. 17.The polymer mixture of claim 1 wherein component (ii) is unhydrolyzed.18. The polymer mixture of claim 1 wherein the anhydride component ofcomponent (ii) is maleic anhydride and the olefinic or styrene componentis selected from one or more of isobutylene, vinyl acetate, ethylene,and styrene.
 19. The polymer mixture of claim 1 wherein component (i) isa grafted product.
 20. The polymer mixture of claim 19 wherein component(i) is grafted onto carboxymethyl-cellulose.
 21. The polymer mixture ofclaim 1 wherein component (i) is present in a fraction in a range from99.7% by weight to 85% by weight, and component (ii) is present in afraction in a range from 0.3% by weight to 15% by weight.
 22. A hygienearticle comprising a polymer mixture of claim
 1. 23. A method of anabsorbing aqueous fluid and reducing odor formation comprisingcontacting the fluid with a polymer-mixture of claim 1.